The present invention relates generally to the field of telephone accessories, and more particularly, to the field of signal amplitude adjustment and impedance matching for compatibility with the various telephone handset port interfaces.
Conventional telephones usually consist of a telephone body and a corresponding handset. One drawback to such conventional telephones, however, is that the telephone handset prevents the user from using his or her hands for other tasks. For example, it is difficult to use a telephone handset while simultaneously operating a computer keyboard. Accordingly, telephone headsets that perform the same communication functions as telephone handsets are now commonly used. With telephone headsets, a user is free to use his or her hands for other tasks while talking on the microphone that forms part of the headset.
A telephone headset typically requires a headset adapter (i.e., interface unit) to correctly couple the headset to the telephone body. The headset and headset adapter are typically manufactured independently of the handset and telephone body. Additionally, it is desirable for the headset adapter to be compatible with many different telephone bodies and handsets. As a result, the headset adapter must be matched with a specific telephone body and handset. In particular, there is a requirement to match the transmit output characteristics of the headset adapter with those of the host telephone handset-microphone. This matching requirement is due to the many different handset microphone technologies that are presently in use and the need to ensure a connection to the Public Switched Telephone Network (PSTN) is compliant with various national regulations and telephone operating company specifications.
Common handset microphone technologies include carbon, electret, dynamic, piezzo, and moving magnet type acoustic transducers. Each of the above microphone technologies has a different output level and a different output impedance. In order to correctly interface or match a headset adapter to the host phone (without causing imbalance problems such as hum, echo, distortion or poor intelligibility), the output characteristics of the headset adapter and host phone must match. Improper adjustment of transmit level can cause distortion which may introduce out-of-band harmonics which could potentially interfere with network signalling, impacting telco functions such as billing and call progressing.
A significant percentage of headset users incorrectly assume that the headset adapter is properly matched with the host phone after they install the headset and hear the dial tone in the headset receiver. Consequently, a user may not bother to adjust the headset adapter transmit output level and impedance so that there is proper matching between the headset adapter and host telephone. The improperly matched headset adapter and host phone will typically result in the above-mentioned imbalance problems.
Furthermore, many users find the headset (and headset adapter) installation process to be difficult and problematic. For example, the user must perform various difficult installation and testing steps so that the headset is properly installed in the host telephone system. Such difficulties not only impact the productivity of the user, but also impart a negative opinion to the user of the headset/handset unit during the impressionable early periods of use.
Moreover, conventional transmit output level adjustment circuits do not automatically adjust the output impedance level for electret, piezzo, and dynamic handset microphones. Thus, longitudinal imbalance problems are introduced into the phone line via the host phone. These longitudinal imbalance problems cause xe2x80x9chumxe2x80x9d and xe2x80x9cbuzzxe2x80x9d noises, echo, radio interference, and other interference that can seriously reduce the intelligibility and productivity of the headset/host-phone system.
Previous solutions for adjusting transmit output characteristics include using manually-operated switches, trim potentiometers, or a combination of both. For example, the M10 headset adapter, which is commercially available from Plantronics, Inc. of Santa Cruz, Calif., requires the user to set the output gain level and output impedance by adjusting a six (6) position slide switch and turning a small trim potentiometer. While the M10 headset adapter primarily provides adjustability for the handset port wiring, the slide switch also permits selection between two output stages. One output stage is for a carbon handset microphone, while another output stage is for the dynamic or electret handset microphone. The transmit level is then adjusted by a separate trim potentiometer.
Another conventional transmit output characteristics adjustment circuit uses a combination of a three (3) position slide switch and a thumbwheel-type potentiometer to adjust the output gain level and output impedance. A disadvantage of this approach is a three-position slide switch limits the number of types of telephone sets that may be compatible with the headset adapter.
Another conventional headset adapter uses a dual in-line package (DIP) switch in conjunction with a screwdriver-adjusted trim potentiometer for adjusting the wiring configuration, output gain level and output impedance. However, the very small size of the DIP switches and trim potentiometer presents difficulties for users and requires the use of special tools when performing the adjustment for the headset adapter.
In yet another conventional approach, the output transmit level is adjusted by use of a slide switch and a computerized customer support system that performs measurements over a phone line and that adjusts the headset adapter remotely. However, this approach disadvantageously requires the user to also perform difficult adjustment steps during the headset installation process and to perform an additional step of calling the customer support system.
The conventional approaches discussed above have proven to be very un-intuitive and have caused difficulties for many customers during the headset installation process. Additionally, a trim potentiometer with sufficient range for matching three different output levels is prone to improper adjustment during installation, thereby resulting in poor performance of the headset/host-phone system. Thus, there is a need for an apparatus and method that eliminates the problems encountered by users during the installation of headsets in various telephone sets with varying microphone technologies.
The present invention provides an adaptive transmit amplifier that includes a detector and a gain/impedance switching circuit, with the detector either controlling the gain/impedance switching circuit directly or via a logic/timing circuit.
The fundamental blocks of the present invention may take on many forms or topologies, and it is the combination of these blocks applied to solve the problem of correct installation of telephone accessories that is one of the unique features of the invention.
The detector is capable of detecting a bias current value in a transmit circuit of the telephone. The detector is also capable of generating an output signal, and the value of this output signal is determined by the bias current value flowing in the transmit circuit. In one embodiment, the detector comprises, for example, an opto-coupler stage that performs switching functions in response to a bias current value in the transmit circuit of the host telephone. In another embodiment, the detector comprises a Hall Effect detector coupled to the transmit transformer of the headset adapter transmit output stage. In yet another embodiment, the detector comprises a pulse detector including a headset/handset relay, a pulse shaping network, and an integrating detector.
The gain/impedance switching stage preferably includes independent gain and impedance switching circuits, the latter circuit actively used in the detection process to test the output impedance of the host telephone bias circuit. In one embodiment, the gain/impedance switching circuit includes analog switches for selecting the gain level or the impedance level of the adapter transmit output stage. In another embodiment, the gain/impedance switching circuit includes solid-state relays such as opto-FETs (optically coupled field effect transistors) or opto-SCRs (optically coupled silicon controlled rectifiers). In yet another embodiment, the impedance switching stage includes an impedance matching network.
In another embodiment, the fundamental blocks of the invention are controlled by the timing/logic block preferably including, for example, a microcontroller for executing a digital program to coordinate the sequence of impedance switching and bias current detection. In another embodiment, the timing/logic block includes a digital signal processor. In still another embodiment, the timing/logic block includes a window comparator.
Thus, the present invention advantageously provides an adaptive transmit amplifier that automatically adjusts the impedance and gain levels of, for example, a headset adapter transmit output stage so that the headset adapter transmit output characteristics match those of the particular handset microphone technology in the host telephone. In particular, the present invention automatically detects the voltage across (and/or current flowing through) the handset transmit circuit in order to automatically match the output characteristics of the handset transmit circuit and the headset adapter transmit output stage. The invention, therefore, eliminates the problems and complexity encountered by users when installing headsets in host telephone systems. As a result, users of headsets will find the headset adapter installation process to be more intuitive and less time consuming.